This invention relates to manufacturing integrated circuits and particularly to a tool and method for butt welding a wire to an integrated circuit frame.
Fabrication of semiconductor devices includes two areas of operations: forming electrical contacts between wires and terminals of components, and packaging, i.e., encasing the device in a plastic or ceramic enclosure to provide mechanical support and protection from the environment.
Contacts between a wire or pin and a pad (terminal) generally fall into two categories: permanent and temporary.
In permanent applications, the requirement is to secure an end of a wire to a pad. The pad is the surface of a lead frame or the terminal of a semiconductor die. Methods used to permanently bond the end of the wire to such terminations include soldering and ultrasonic welding.
Contacts of a temporary nature include contacts of conductors to terminals of a test circuit where contact is maintained only during a brief test procedure. In this application, the test wire is commonly supported in a manner wherein the wire maintains a resilient (springlike) force against the terminal of the device under test.
Numerous techniques for forming permanent and compressive bonds have been disclosed in the Patent literature.
U.S. Pat. No. 3,460,238 discloses a technique whereby the wire severing operation comprises moving the needle with holding pressure sufficient to frictionally engage the wire and insufficient to deform the wire away from the bond area.
U.S. Pat. No. 3,73,481 to Lins et al discloses forming pin-like gold pedestal structures atop terminal portions of an n integrated circuit device by thermocompressing gold spheres and shaping the spheres with a heated vacuum holder.
U.S. Pat. No. 5,095,187 to Gliga discloses a wire-bonding technique involving a combination of heat, pressure and vibration. This patent discusses weakening or severing the wire by localized application of heat and how the severing operation may result in a broadened portion on the severed end of the wire
U.S. Pat. No. 6,062,459 to Sabyeying discloses a wirebond clamp for holding fingers of lead frames against a heater block. The clamp includes a base plate having an array of apertures for the lead wires.
U.S. Pat. No. 6,336,269 to Eldridge et al discloses a flexible elongate member (such as a gold wire) mounted on a contact area and is configured to have a springable shape, such as a wire stem having at least one bend.
Packaging is an important part of semiconductor technology wherein the object of the package is to confer mechanical stability and protection from the environment on a circuit comprising a plurality of electrical connections between electronic components.
Ceramic packages are required where long life of the encapsulated device is required. Plastic packaging of the circuit is practiced where cost is an important consideration with some degree of mechanical stability and protection from the environment.
Intensive research into packaging techniques has been directed toward the development of polymer composites that have improved thermal resistance, lower coefficients of thermal expansion, reduced moisture absorption characteristics. More recently efforts have been directed toward the development of three-dimensional packages. Numerous patents are directed toward these efforts.
For example, U.S. Pat. No. 5,985,693 to Leedy is directed toward fabrication of integrated circuits from flexible membranes formed of very thin low stress dielectric materials and semiconductor materials.
U.S. Pat. No. 6,147,335 to Von Arx discloses various components of thermal systems molded within a polymer sheath. The use of transfer and compression molding allows for the use of thermoset polymers containing very high levels of reinforcement fillers.
In a related configuration, it is required to form electrical connection of the ends of wires to respective fingers of a lead frame. Attempts to form such contacts involve forming a permanent bond of the end of a wire or pin to the broad surface of the finger. Encasing the contact in a plastic after the contact is formed is difficult to carry out because of the mechanical forces exerted on the contact during the subsequent molding process.
Ultrasonic bonding the tip of a wire or pin to the broad surface of a lead of a lead frame requires that the surface of the lead be scrupulously clean in order to obtain a strong bond.
It is an object of this invention to provide a tool and process for forming a permanent bond of the tip of one or more wires or pins to the surface of a metal sheet.
It is a further object of the invention that the contact(s) be encapsulated in a resin medium.
It is a further object that, in forming the contact to the surface of the metal to which the tips are bonded the surface of the metal be devoid of contaminating films including oxidized films that would diminish the strength of the bond. It is contemplated that a particular application of the method and tool be to the formation of wires and pins bonded to the leads of a led frame.
This invention is directed toward a process which begins with cleaning the surface of the lead frame and then providing a clamp
The clamp has any one of several embodiments.
In one embodiment, the clamp has one jaw that is a plate having a flat clamping surface facing another other jaw being a (preferably round) bar. The clamping surface of the bar, (i.e., the end of the bar facing the clamping surface of the plate) has a raised rim around its circumference.
In another embodiment, the one jaw is also a bar (instead of a plate) having a flat end surface and the two bars (aws) are aligned.
According to various versions of the invention, either or both jaws are moveable toward and away from one another.
When the sheet is clamped between the clamping surfaces, a casting medium is forced under high pressure into the cavity surrounding the jaws clamped together. The clamping force between the jaws is concentrated at the ridge of one jaw toward the clamping surface of the second jaw and is generally sufficient to indent the surface of the metal between the jaws. In practice, the pressurized encapsulating medium is forced into the cavity, which includes the jaws in contact with one another. However, the ridge) prevents the encapsulating medium from leaking into the interface between the clamping surfaces of the jaws.
When the clamp (bar and plate} are withdrawn from the casting, there is left a package including the lead frame with attached lead encapsulated in the casting medium. An open column leads from the surface of the lead to the outer surface of the package. The surface of the lead has been kept free of encapsulating by virtue of the configuration of the clamping surfaces in accordance with the invention.
In a second step of the process, a wire (or pin) is inserted into the open end of the column and into contact with the surface of the lead frame on the opposite end of the open column.
Ultrasonic energy is then applied to the wire so as to ultrasonically bond the wire to the lead frame.
The open column enclosing the bonded wire is then impregnated in a second encapsulating step so as to seal the wire in position welded to the lead frame thereby completing the packaging operation. A crucial feature in the foregoing process is the use of the hold down bar with the raised rim around the edge that effectively prevents the liquefied encapsulating medium from contaminating the surface of the lead which would otherwise interfere with formation of the bond between the surface of the lead and the wire tip.